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A biplane is a fixed-wing aircraft with two main wings. The first powered heavier-than-air
aircraft, the Wright brothers' Wright Flyer, used a biplane design, as did most airplanes in
the early years of aviation. While a biplane wing structure has several advantages it inevitably
produces much more drag than a similar monoplane wing. Improved structural techniques and
materials, and the need for greater speed, effectively made the biplane configuration obsolete
for most purposes by the late 1930s.

The term is also occasionally used in biology, to describe the wings of some flying animals.

In a biplane aircraft, two wings are placed one above the other. Both provide lift. Very
often there is a fuselage to which the lower wing is attached, while the upper wing is raised
above, although other combinations have occurred. Almost all biplanes also have a third
horizontal surface, the tailplane, to control the pitch, or angle of attack of the aircraft
(although there have been a few exceptions). Either or both of the main wings can support
flaps or ailerons to assist lateral and speed control, although the upper wing has tended
historically to be more important in this regard. Often there is bracing in the form of wires
and slender struts positioned symmetrically on either side of the fuselage.

Variations on the biplane include the sesquiplane, where one wing (usually the lower) is
significantly smaller than the other, either in span, chord, or both. Sometimes the lower
wing is only large enough to support the bracing struts for the upper wing. The name means
"one-and-a-half wings". Another (aerodynamically quite distinct) variation is the tandem wing
biplane, which is an aircraft with one wing in front of the other (e.g. a wing in the nose and
a wing in the tail).

Advantages and drawbacks to biplane designs

RAF BE2c biplane of 1915.Aircraft built with two main wings (or three in a triplane) can usually
lift more than can a similarly sized monoplane of similar wing-span. Another advantage of biplane
wings is that a given wing area requires a shorter wing span, which tends to afford greater
maneuverability. The struts and wire bracing of a typical biplane form a box girder that permits
a light but very strong wing structure.

On the other hand there are many disadvantages to the configuration. Each wing negatively
interferes with the aerodynamics of the other. For a given wing area the biplane produces more
drag and less lift than a monoplane, but this effect can be reduced by placing one wing forward
of the other. Placing one wing forward of the other is known as stagger. Forward stagger (where
the upper wing is further forward) is most common, but backward stagger has also been used,
notably in Beechcraft Staggerwing. Excessive amounts of stagger distort the box girder
effect of the wing - and this tends to reduce the structural benefits of the biplane layout.

History

Biplanes were most successful in the early days of aviation when the all wing structures
(including those of monoplanes) needed to be strengthened by external bracing wires and struts.
The inherent strength of the biplane configuration in this situation was decisive, as early
monoplanes tended to suffer from overly flexible wings, resulting in poor lateral control, and
a greater risk of wing failure. Once the need for external bracing was removed by the widespread
adoption of improved structural materials and techniques monoplanes quickly superseded biplanes
in aviation. Modern biplane designs now exist only in specialist niche roles and markets such as
aerobatics and agricultural aircraft.

The vast majority of biplane designs have been fitted with reciprocating engines of comparatively
low power; exceptions include the Antonov An-3 and WSK-Mielec M-15 Belphegor, fitted with turboprop
and turbofan engines, respectively. Some older biplane designs, such as the Grumman Ag Cat and the
aforementioned An-2 (in the form of the An-3) are available in upgraded versions with turboprop engines.

In an interesting parallel to the role of the biplane in human aviation, some researchers
have suggested that the feathered dinosaur Microraptor glided, and perhaps even flew, on four
wings which were held in a biplane-like arrangement. This was made possible by the presence
of flight feathers on both the forelimbs and hindlimbs of Microraptor, and it has been suggested
that the earliest flying ancestors of birds may have possessed this morphology, with the monoplane
arrangement of modern birds evolving later.[1]